FIG. 1 partially and schematically shows photodiodes, side by side, of a network of photodiodes, for example forming an image sensor. Each photodiode comprises a semiconductor area 1 for converting photons into electron-hole pairs. A complete photodiode comprises junctions (not shown) between semiconductor regions of opposite type to store electrons, and various read transistors for transferring electrons.
More specifically, FIG. 1 illustrates a network of BSI-type (Back Side Illumination) photodiodes. In this type of structure, semiconductor areas 1 currently have a thickness no greater than from 2 to 5 μm. Photodiodes 1 are separated by insulating regions 3, currently called DTI (Deep Trench Isolation) in the art. Under the semiconductor layer containing photodiodes 1 are formed various metallization levels 4 embedded in a succession of insulating layers generally designated with reference numeral 5. The assembly rests on a support or handle 7, currently a silicon wafer. On the upper side or back side of the structure, each of the photodiodes is topped with a filter 9 corresponding to the color to be detected by this photodiode. The red, green, and blue filters are designated with references R, G, and B. Each of filters 9 is topped with a microlens 10. Filters 9 and the upper surface of semiconductor layers 1 generally have a thin insulating layer 11, or an assembly of insulating layers forming an antireflection layer, provided between them.
PCT patent application WO2012/032495 (incorporated by reference) discloses that the introduction of light into the photodiode raises an issue when the lateral dimensions of the illuminated surface of a photodiode are very small, in the order of wavelength λ of the light that the photodiode is intended to capture, or operating wavelength. Thus, such photodiodes of very small dimensions have a low quantum efficiency. The PCT patent application provides, to increase the quantum efficiency of the photodiode, arranging on the upper surface thereof a pad having lateral dimensions smaller than the lateral dimensions of the photodiode.
The PCT patent application specifies that the concerned photodiodes are of FSI or Front Side Illumination type. In this case, specific problems are posed due to the fact that the metallization network is on the side of the illumination source. As will be seen hereafter, the structure provided in the PCT patent application has disadvantages in the case of BSI-type photodiodes.
Thus, a problem arises to absorb the maximum possible number of photons in structures such as that shown in FIG. 1, in the case of pixels of small dimensions, that is, having lateral dimensions in the range from one to five times the operating wavelength. More generally, the smaller the collection area, the more the quantum efficiency of the device is a problem. This problem more or less arises in all semiconductor photodiodes.
It should thus be noted that in photodiodes of very small dimensions, an increase, even low, of the quantum efficiency or absorption rate of the useful portion of the photodiode is in practice extremely important for the detection of low-intensity light. Thus, an efficiency gain from 1 to 5% will be considered as a significant gain by the user.